Hot pressing has typically been used to densify powder materials under pressure and heat, the pressure assembly typically being comprised of close fitting members constructed of carbon, preferably graphite. Pressure is applied to the movable parts of the assembly by hydraulic means, and heat is applied by placing the assembly in a heated environment, such as a furnace, which introduces heat by induction heating or resistance heating (see U.S. Pat. No. 3,455,682).
One of the most significant problems associated with hot pressing of certain ceramic crystalline powders is the tendency of the powder to react and adhere to the mold or container as a result of the ambient environment or conditions prevailing in the compacted powders and the high temperature and pressure applied through the pressing assembly in certain zones thereof. This results in damage to the assembly walls as well as to the surfaces of the molded body by the formation of a reaction layer. Particularly, when attempting to hot press silicon nitride, there is a tendency for a reaction to take place between the silicon nitride and carbon and carbon monoxide (the latter is released as a result of the high heat and available oxygen content of the graphite assembly) to form silicon carbide along with small quantities of free silicon and oxides of nitrogen and/or free nitrogen and oxides of carbon as well as oxides of silicon. Porosity in the Si.sub.3 N.sub.4 is also produced. The silicon carbide reaction layer makes the use of the die walls unusable for subsequent hot pressing because it destroys the dimensional relationship as well as chemical inertness of the pressing assembly. In addition, SiC and other reaction products must be removed by diamond grinding from the Si.sub.3 N.sub.4 end products, which is an expensive step.
The prior art has concerned itself with the reaction between metal casting vessels formed of graphite and molten silicon which has been inserted into such molds. The art has typically utilized alkali fluorides or silicates to coat the graphite and prevent an undesirable chemical reaction (see U.S. Pat. Nos. 4,234,630; 3,023,119; and 2,840,480).
The prior art has also utilized release agents as materials to facilitate separation and removal of the hot pressed silicon nitride from graphite assemblies after hot pressing. The release agents have typically been comprised of boron nitride or an anisotropic graphite of low density. The graphite is in an insertable form, such as foil, and possesses low permeability and an exceptionally low coefficient of friction. These materials, either as a coated slurry or as an insert sheet or foil, are generally placed on the contact surfaces of the graphite walls of the pressing assembly and also on the walls of the silicon nitride material if it happens to be a compacted body prior to being inserted in the pressing assembly. Unfortunately, such release agents are not effective and do not preclude the formation of silicon carbide between the interface of the silicon nitride body and the graphite walls, even though the boron nitride or graphite pieces are interposed therebetween. Thus, again, the graphite die assembly walls are unable to be reused in subsequent pressing operations (see U.S. Pat. Nos. 3,892,835 and 3,819,787 which are pertinent to the use of boron nitride, and see U.S. Pat. No. 4,113,830 pertinent to the use of an insertable graphite sheet).